Hematopoiesis occurs through the fine regulation of self-renewal and differentiation of the pluripotent hematopoietic stem cell (HSC). This occurs in a complex microenvironment, with many factors playing possible roles in the regulation of hematopoiesis. The identification of regulators of HSC self-renewal is especially important for clinical applications involving in vivo and ex vivo expansion of HSCs for purposes such as HSC transplantation. This application is based on recent novel discoveries of the roles of morphogens, in particular members of the Notch family and vitamin A receptors (RARs), in hematopoiesis. Notchl has been well described as being a regulator of HSC self-renewal. We have recently shown that the vitamin A derivative, all-trans retinoic acid (ATRA) enhances the self-renewal of murine HSCs, and that RAR gamma signaling is critical for these effects. We have also shown that RAR gamma significantly impacts on Notchl expression, indicating a link between these morphogens. Interestingly, morphogens also regulate the formation of bone cells, including osteoblasts, which have recently been described as being a key component in the HSC niche; hence morphogens also potentially regulate the HSC niche. This application aims to further explore interactions of morphogens in regulating HSC self-renewal both in isolation of the HSC niche and in a context-dependent manner with respect to the HSC niche. We will use the mouse model in these studies, combining in vitro cellular and molecular biology techniques to determine the interactions between morphogens and their effects on HSCs. In vivo transplant studies will be performed to assess the effects of these morphogens on HSC self-renewal. The aims of this project are therefore to: 1) investigate the interactions between morphogens known to regulate HSCs independent of the HSC niche. 2) determine the impact of morphogens on the HSC niche in a context-dependent manner. Lay: In recent years, there has been a desire to increase the numbers of stem cells that are capable of producing all blood cells for therapeutic purposes such as improving stem cell transplants. Previous attempts to increase the numbers of these cells both outside of and within the body have not been very successful. Our data suggest that a vitamin A compound may be very useful in improving both of these approaches. [unreadable] [unreadable] [unreadable]